High-Performance-Rotor Aerodynamic Testing

Abstract
A whirl and wind tunnel test was conducted to measure performance and stall vibration benefits associated with a novel Retreating Side Blowing (RSB) concept, Miniature Trailing edge Effectors (MiTEs), and Lateral Lift Offset (LLO) trim. A new 28 ft diameter rotor mounted to an articulated S-76R rotor hub was flown at the National Full Scale Aerodynamics Complex (NFAC). Technical objectives were slightly improved performance and significantly reduced retreating blade stall vibration at high blade loading-advance ratio combinations. The blades were well instrumented with five fully strain gauged spanwise locations as well as external, internal, and blowing slot pressure and temperature sensors. Retreating blade stall conditions were reached between 0.24 to 0.61 advance ratios. The RSB pumping power and slot profile drag effects reduced L/De at mid blade loadings but leading edge blowing regained the performance loss at high blade loadings. The MiTEs showed a slight L/De improvement at high blade loadings. Lateral lift offset showed up to 35% L/De improvements as blade loading was increased. The RSB concept slightly reduced retreating side torsional vibration. The RSB dynamic lift and moment improvements were not as beneficial as expected due to the dynamic Mach number on the retreating side. The MiTEs reduced retreating blade stall vibration by 20% to 40%. Test data revealed rotor blade peak torsional vibration was not improved above advance ratio 0.32 by any concept. This is because peak torsional blade loads were not generated by retreating side stall. They were generated roughly between 20 deg and 200 deg azimuth positions. Therefore even if the retreating side technologies worked perfectly the rotor stall boundary would not have improved. Subsequent review of UH-60A slowed rotor and AH-56A flight test data show the same phenomena.